The present invention relates to apparatus and a method for measuring the temperature profile across the width of a strip of moving material, e.g. hot steel strip. By way of background to the invention, one example of its practical utility in a hot strip mill will be explained, although the invention is not restricted to such use.
In hot strip mills, it is known to monitor the thickness profile of the strip, in order to effect corrective adjustments of the roll stands when necessary, by projecting a fan beam of X or gamma rays through the strip to a row of detectors extending across the width of the strip. See our U.S. Pat. No. 4,047,036 for details of such a system. The intensity of radiation received at a detector is given by the following equation: EQU I=I.sub.o e.sup.-upx,
where:
I=received intensity PA1 I.sub.o =incident intensity PA1 u=mass absorption coefficient PA1 p=density PA1 x=thickness. PA1 x'=cold thickness, PA1 T=temperature, and PA1 A and B are constants.
Since p is a function of temperature, namely an approximately linear inverse function, the thickness x indicated by measuring I is influenced by temperature. The hotter the strip, the thinner it appears. However, it is the "cold thickness", i.e. the thickness of the strip when cold, that is normally required. It is known to derive this by measuring the temperature of the strip and calculating its cold thickness from its measured thickness using an equation of the form: EQU x'=A.times.(1+BT) (1)
where:
However, a single measured temperature does not lead to satisfactory compensation because there is significant temperature variation across the width of the strip metal which naturally cools more rapidly at the edges of the strip. There can also be cool bands away from the edges, arising from preferential cooling of the billet before entry into the mill stands. It would therefore be desirable to measure the temperature profile so that there is a separate value of T for correcting each thickness measurement, or at least sufficient values of T to provide a sufficiently accurate measurement for each detector in an arrangement where each value of T pertains to a band of the strip covering a group of the detectors.
It is possible to make temperature measurements using a diode array. The use of radiation responsive diodes to measure temperature is known and arrays of diodes are available which respond to infra-red radiation emitted from bodies above around 700.degree. C. However such diodes do not provide an inherently accurate measurement and need to be calibrated against a standard such as a radiation pyrometer. The number of diodes in an array must be very large, say 1000 diode elements, and calibration of so many diodes presents problems. Instead of diodes, a charge coupled device (CCD) may be used. Again calibration is necessary.